The Bible (Esther 8:10, 8:14) references an animal called האחשתרנים (the Achashteranim). I have seen Jewish Medieval commentators, including R. Joseph Kara and R. Elazar Rokeach of Worms (unavailable online) who explained that this refers to eight-legged animals. However, both of those works were first published in the last few decades. I also found that R. Jacob Emden (1697-1776) in his glosses to the Talmud (Megillah 18a) writes the same thing (see here) in the name of "writers of the provinces," which seemingly refers to non-Jewish chroniclers. It is unlikely that Emden's source are those Jewish Medieval commentators who I mentioned because their works were published after his lifetime. So what is the source that Emden used for this?
In Esther 8:10, 14 the King James Version of the Bible says that the letters were sent "by posts on horseback, and riders on mules, camels, and young dromedaries." The New International Version, New Living Translation, and English Standard Versions all say fast horses specially bred for the king.
External sources have this to say:
In Es 8:10,14 we find the doubtful words (8) 'achashteranim, and (9) bene ha-rammakim, which have been variously translated. the King James Version has respectively "camels" and "young dromedaries," the Revised Version (British and American) "used in the king's service" and "bred of the stud," the Revised Version margin "mules" and "young dromedaries." https://www.internationalstandardbible.com/H/horse.html
Another source makes this comment:
achashteranim (Esther 8:10,14, the King James Version "camels," the American Standard Revised Version "that were used in the king's service".
There are a few unusual words which have been translated "camel" in text or margin of one or the other version. (See list of words at beginning of the article) Bekher and bikhrah clearly mean a young animal, and the Arabic root word and derivatives are used similarly to the Hebrew. Rakhash, the root of rekhesh, is compared with the Arabic rakad, "to run," and, in the Revised Version (British and American), rekhesh is translated "swift steeds." Kirkaroth, rammakhim and 'achashteranim must be admitted to be of doubtful etymology and uncertain meaning. Alfred Ely Day https://www.biblestudytools.com/dictionary/camel/
There may be a mythological eight-legged animal, however. Rav Yaakov Emden in his glosses to Megillah 18a, says that he saw in Persian history books that this is a unique breed of camels with two humps and eight legs, that run very quickly.
Can we identify what sort of camel Rav Yaakov Emden was discussing? I think we can. I believe he was referring to the Sarabha. From Essays on Indo-Aryan Mythology (and thus that of Persia):
Thus, the word Sarabha was used for real animals, such as peacocks, wild buffalos, young elephants, and camel. There is also the "fabulous" Sarabha, which is ashtapad, having eight legs, four of which are upward.
I don't know if in any of these texts, it is both eight-legged and a camel. It is certainly possible; it is also possible that Rav Yaakov Emden was relying on some text that conflated the normal Sarabha, a camel, and the fabulous Sarabha, which was eight-legged. Or else it spoke of Sarabha in different contexts and Rav Yaakov Emden put two and two together....
I think, therefore, that Rav Yaakov Emden was misled, and that this creature is surely not the same one as in the pasuk in Esther. This is one of the pitfalls of Torah UMaddah -- where the madda fails, the peshat fails as well.
He is likely on the right track, though, in saying that this is some sort of camel. After all, the typical Sanskrit word for camel is ushtra. Compare with achastranim. This is therefore likely some sort of fast camel, bred for its swiftness. http://parsha.blogspot.com/2010/07/rav-yaakov-emdens-eight-legged-camel.html
I hope you find this useful.